This project focused on the development of multiscale models for molecular crystals, with special emphasis on the secondary explosive cyclotrimethylene trinitramine (RDX). Physical understanding related to the molecular scale mechanics of this material was also developed. This data is used as starting point for the definition of physically-based coarse grained models. In turn, these models are to assist the design of new energetic materials with enhanced energy release rates and reduced sensitivity to unintentional detonation. The following results have been obtained: a) we investigated conformational stability of the RDX molecule in the crystal as a function of temperature and determined the density of conformers in steady state at temperatures between room and melting, b) we identified the stable dislocations in RDX and, for the first time, established a ranking of slip systems in terms of the Peierls stress (critical stress for dislocation motion), c) we discovered a family of point defects which are rotated and distorted molecules embedded in an otherwise perfect crystal; the stability of such point defects was demonstrated with DFT simulations and a method was proposed to detect them experimentally using Raman spectroscopy, d) we are currently investigating the effect of temperature on the ranking of slip systems, which is a result needed in crystal plasticity models of hot spot formation, e) we developed a family of coarse grained models for the RDX crystal in which the molecules are gradually coarse grained to higher and higher degrees, ranging from full atomistic detail to rigid blobs, and we determined the level of error introduced by coarse graining in various macroscopic measures of crystal thermo-mechanics.